Insensible water loss should probably be called immeasurable water loss. As a resident we all learned to order I&O (Intake and output of fluids) and daily weights as a way to monitor the change from day to day in our hospitalized patients fluid status. Rapid weight gain, especially combined with a higher fluid intake than urine output was often a clue that we needed to watch for problems associated with fluid retention like congestive heart failure, pulmonary edema, and edema. We also learned in med school, and this was apparent in clinical care of patients, that everyone loses fluids constantly in ways other than urine formation and output. Some of these ways are measurable using not-too-inconvenient or impractical means. We could measure or roughly estimate fluid lose in emesis, diarrhea, blood loss, drainage of body fluids from various drains placed, and still we knew that these cannot account for all of the water a patient ingests or has infused.
The water loss of patients that is not able to be measured in any practical way is commonly referred to as insensible water loss, or fluid loss. Although there are numerous means of insensible water loss, respiration is by far the largest in patients who are not sweating profusely. Each breath of air we inhale usually contains less than 100% humidity. The air is humidified in the nose, sinuses, and upper airways so that the air that reaches the lungs, and is subsequently exhaled is essentially 100% humidity at body temperature. The carrying capacity of water for air goes up rapidly as the body temperature goes up, so respiratory loss of patients with fever is higher than the respiratory loss of afebrile patients.
Sweating is another means of insensitive water loss. Sweating obviously varies with ambient temperature, body temperature, physical activity and other variables. We often see patients who are ill in the hospital or at home have profuse diaphoresis and the water loss from this may be significant.
What physicians call “third spacing” of fluids is a shift of body fluids into an area neither inside cells nor in the vascular system. Inflamed tissues can swell leading to a shift of significant amounts of body fluid into areas where the fluid does not support the vascular system. In some cases this can be rapid and in large volume, leading to shock and vascular collapse. An example is acute pancreatitis where large amounts of edema can occur rapidly in the inflamed tissues in the abdomen and retroperitoneum. Another situation is when the small bowel stops functioning to move the fluid contents through to the colon where fluids are generally reabsorbed. In this situation, called an ileus, large amounts of fluid can accumulate in the gut, another third space where fluid can be lost to the vascular system. These situations can require significant IV fluids to maintain blood pressure, renal perfusion, urine output, and to support the patient’s metabolism.
For the kidneys to function well they require water to excrete as urine. The kidneys of a healthy person can concentrate urine only to a certain degree, after which they cannot both continue to function and preserve water needed in the body. With inadequate fluid intake to replace urine loss, insensitive fluid losses, and fluid functionally lost in third space accumulation, a person becomes unable to maintain blood pressure and renal perfusion at a level that supports kidney glomerular filtration. When this happens the kidneys can acutely fail, in a condition called acute tubular necrosis. If this is brief and good fluid resuscitation occurs, the kidneys can recover. If prolonged renal failure from acute tubular necrosis can be permanent.
A need for water that is not obvious is that the requirement for water actually increases with increased nutrition. The waste products of the burning of calories, especially high protein foods, requires water for the needed urine output. Although tiny amounts of water are actually produced as end products of the burning of carbohydrate foodstuffs, in general increased caloric intake requires a modest increase in water intake for homeostasis.
In summary of the fluid taken in by oral route or from IV fluid infusion either accumulates in the body or is lost from the body in one way or another. We refer to the losses as sensible, i.e. through ways we can measure, or insensible through ways like respiration, water loss in the stool, and sweating. Functional loss of fluids into body areas outside the vascular system and outside the body’s cells is called third space losses. Although this is fluid that eventually may become absorbed into the vascular system and utilized, it is functionally lost for use to maintain normal bodily functions.
So when you are looking at whether you are taking in enough water the best single indicator is your urine output. If you are not voiding at least 2-3 times a day in moderate amounts, you need to drink more. If our patients are not putting out 30-50 ml of urine an hour, we want to try to figure out why not, and try to take measures to remedy the situation.
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